1. Field of the Invention
The present invention relates to a vertical heat treatment apparatus and a method of transferring the substrates to be processed, and more particularly to a vertical heat treatment apparatus and a method of transferring the substrates to be processed that make it possible to transfer a plurality of to-be-processed substrates at a time to a holder having a ring-shaped support plate.
2. Description of the Related Art
Semiconductor devices are manufactured by performing various heat treatment processes, such as oxidization, diffusion, CVD, and annealing, on semiconductor wafers or other substrates to be processed. A vertical heat treatment apparatus capable of performing a batch process, that is, thermally processing a large number of wafers at a time, is used as a heat treatment apparatus for performing the above processes.
The vertical heat treatment apparatus includes a thermal reactor, which has a reactor opening at its bottom; a cover, which hermetically closes the reactor opening; a holder (also referred to as a wafer boat), which is positioned above the cover to hold a large number of wafers via a ring-shaped support plate in such a manner as to arrange the wafers vertically at predetermined intervals; an elevator mechanism, which moves the cover up and down to load the holder into and unload the holder from the thermal reactor; and a transfer mechanism, which has a transfer plate (also referred to as a fork) for transferring the wafers between the holder and a storage container (also referred to as a cassette or hoop) that stores a plurality of wafers at predetermined intervals. The ring-shaped support plate is used to inhibit or prevent the generation of a slip (crystal fault) at the periphery of a wafer during a high-temperature thermal process.
An upthrust type transfer mechanism (hereinafter referred to as transfer mechanism A), which includes a main transfer plate 50 and an upthrust transfer plate 51 as shown in FIGS. 10A to 10C, is employed as the transfer mechanism (refer, for instance, to JP-A-5-13547). The main transfer plate 50 is made of a plate-like body having an upper surface that comes into contact with the lower surface of a wafer w. The upthrust transfer plate 51 is made of a plate-like body having three support pins 52 that come into contact with the lower surface of a wafer w on the upper surface thereof.
When a wafer is to be transferred to the holder, the main transfer plate 50, which supports the wafer w, is inserted into a position above a predetermined support plate 15 in the holder 9. Further, the upthrust transfer plate 51 is inserted into a position below the support plate 15 (FIG. 10A). Next, the upthrust transfer plate 51 ascends to lift the wafer w up from the main transfer plate 50, thereby causing the main transfer plate 50 to leave the holder 9 (FIG. 10B). Next, the upthrust transfer plate 51 descends to let the support plate 15 support the wafer w, and then leaves the holder 9 to complete a transfer operation for one wafer (FIG. 10C).
Another known transfer mechanism (hereinafter referred to as transfer mechanism B) includes a plurality of locking members that are locked onto the lower side surface of a wafer periphery to support a wafer while it is hanging down (with its top grasped). Each locking member is allowed to reciprocate between a wafer support position, at which the wafer is supported while it is hanging down, and a wafer release position, at which the locking member moves to a location outside the outer periphery of the wafer to release the supported wafer. Further, each locking member is driven by an actuator to reciprocate between the wafer support position and wafer release position (refer to JP-A-2003-338531).
However, the conventional vertical heat treatment apparatus spends a considerable amount of time in wafer transfer because transfer mechanisms A and B, which have been described above, can transfer one wafer at a time. In addition, the employed transfer mechanism structure requires a long interval (approximately 16 mm) between the holder support plates (this interval may also be referred to as the transfer interval). This imposes a limit on the batch processing volume because the holder can only accommodate up to approximately 50 wafers. It means that the processing volume cannot readily be increased by the use of the conventional vertical heat treatment apparatus. Transfer mechanism A, in particular, transfers one wafer at a time and takes a considerable amount of time to complete a wafer delivery (which involves more than one operation). As a result, the transfer process involves a long period of time, thereby making it impossible to provide increased throughput.